Bone Graft: How does the broken bone assimilate the graft?

After fractures and bone grafts heal the tissue between the
fracture fragments is "woven bone" or "hard callus". Bone spicules
have been laid down at random and the tissue may be bulkier but is not as mechanically
sound as normal bone. The process of remodelling turns this tissue into "normal"
bone, that is a cylinder with compact bone round the circumference and spongy bone or a
cavity in the middle. In some way, likely related to piezo-electric current generation,
bone that is stressed is stimulated to grow denser and stronger. Unstressed bone is
absorbed. So there is a gradual process of thickening the parts of the callus which are
taking the weight and thinning of the parts which aren't. Over many months this transforms
the injury site back into something which is close to the original state.
In summary the healing process after bone grafting is the same as healing after fracture
but the presence of the bone graft fragments pushes it along faster.
The implications of this process are what I wanted to stress to you all.
1. Once bone healing by hard callus has been accomplished the next stage REQUIRES weight
bearing. But the bone is still at risk of refracture because it is not as strong as normal
bone. Thus we have the paradoxical situation of still protecting the bone but urging
weight bearing; walk, swim, ride bicycles but don't fall over ;-)
2. The process of remodelling is an active metabolic process and requires more of the
body's resources being devoted to that area than usual. The result is persistent swelling,
throbbing and aching from the biological factory which is still in full swing.
3. It all goes on for a long time. I don't have to tell the group that. But I hope it
helps to know just why, and what is happening for all those months.